1. Field of the Invention
This invention relates generally to improvements in variable power dividers and more particularly, but not by way of limitation, to a mechanical variable power divider for use at microwave frequencies that is particularly adapted to adjust the distribution of RF power between two branches of a network, such network most typically being an antenna system on a communication satellite.
2. Description of the Invention
A need for variable power dividers has been well established in communication satellites. Typically, a modern satellite communications system requires that the antenna provide a number of separate beams operating on the same frequency, typically called "frequency reuse", and have means for switching or reconfiguring the beams to accommodate changing traffic patterns. Such changes in switching or reconfiguring the beams may occur frequently or infrequently depending upon the particular location and category of the traffic being handed.
The present invention provides means to alter the distribution of RF power in a microwave circuit between two paths under mechanical control. This alteration is accomplished with minimum losses, and over a wide band of frequencies (consistent with the bandwidth of the microwave circuit, which is usually a waveguide).
The invention is particularly useful in antenna feed distribution networks to permit adjustment of the distribution of RF power between two branches of the network, and conversely in receiving systems, the amount of power contributed by each branch. Such adjustment is required, typically, in an antenna system positioned on a communication satellite in order to alter the beam location and contour, upon command, to accommodate traffic requirements.
It also has the advantage of requiring very little power to operate and requires no power in standby mode. An additional advantage is that it is virtually independent of temperature variations, which is a great advantage over other devices using ferrite materials in either Faraday rotators or phase shifters used for similar purposes. It also has very low RF losses.
The following patents, while of interest in the general field to which the invention pertains, do not disclose the particular aspects of the invention that are of significant interest.
German Pat. No. 863828 dated Jan. 19, 1953 discloses the use of a spiral web of fixed geometry to rotate the plane of polarization of a signal in a wave guide in order to control the relative amounts of radiation from slots in the side of the waveguide. Such a fixed arrangement permits the design of an antenna array in which different segments have different amounts of energy. This patent does not disclose a web contained within a waveguide and having variable and controllable geometry to vary the plane of polarization to control the RF energy.
French Pat. No. 1,137,228 dated May 27, 1959 discloses a spiral septum or web of fixed geometry mounted on a rotatable shaft in a cavity having an "hourglass" shape. Rotation of the shaft moves the part of the septum in the narrow region of the hourglass shape in such a way as to vary the effective dimensions and, hence, change the resonant frequency. As before, the feature varying the shape of a spiral web within a circular waveguide to change the plane of polarization of RF energy a predetermined amount for the purpose of varying RF power is not seen.
U.S. Pat. No. 2,628,278 issued on Feb. 10, 1953 to John F. Zaleski discloses a device for rotating microwave energy. The device utilizes a rotary joint to orient a flexible septum positioned within a hollow waveguide. The rotary joint is rotated to very the septum to a fixed position for the purpose of rotating the RF energy of a fixed single input port to a fixed single output port without loss of RF energy. The purpose of this joint is to be able to position various system components with respect to each other with a minimum of waste space or to apply energy to or derive energy from a movable antenna structure. There is no intention to provide a variable power divider to divide power between two output ports.
U.S. Pat. No. 2,985,850 issued to Carl F. Crawford, et al. on May 23, 1961 shows a variable, highpower microwave power divider wherein a series of ganged individual rectangular waveguide sections are manually rotatable, by manipulation of a handle, relative to one another to create a rotated wave which is shared by two output ports. This device is mechanically complex and inherently lossy. The present invention employs only one single fixed circular waveguide while the Crawford, et al. device uses a plurality of waveguides and rotary joints, which multiplication of parts inherently increases a chance of failure in space.
U.S. Pat. No. 3,024,463 issued on Mar. 6, 1962 to Alvin W. Moeller, et al. for a feed assembly for circular or linear polarization uses a series of rotatable joined circular waveguide sections for adjusting the relative phase of a circularly polarized horn so as to produce a linearly polarized wave at a rectangular waveguide single port. As in the Crawford patent, the series of rotary joints is relatively inefficient, lossy and does not provide a variable power divider.
U.S. Pat. No. 3,296,558 issued to William J. Bleackley on Jan. 3, 1967 relates to a polarization converter comprising metal rods mounted on a torsion wire that twists when rotated. Bleackley provides a rotary joint in the outer wall of the waveguide that can be rotated to twist the torsion wire to determine the plane of polarization of the wave at a single output port. It does not disclose a variable power divider that uses a variable septum positioned within a circular hollow waveguide that may be varied without the use of external rotary joints.